Thermal print heads are extensively used in many types of image generating devices such as printers, facsimile machines and copying machines. A print head of this type has many problems which must be solved before a practical device can be obtained. One is that of interfacing to the print head, which has a large number of elements. A facsimile machine, for example, typically has a print head with a approximately 1800 elements extending in a line. As the paper travels across these elements, they are selectively turned on and off to darken corresponding areas of the paper. Hence, an interface must be made to all of the elements. Moreover, this interface must be of the shielded type, in order to minimize the emissions that radiate from the print head. If too much noise is allowed to enter the system from the radiation from the print head, the signal-to-noise ratio may become degraded to the point that an unacceptable image is produced by the facsimile machine. However, the delicacy of the print head and the necessity for many connections makes shielding of this print head relatively difficult. Moreover, the delicacy of these print heads has caused difficulty in precise alignment of its elements. Another related problem is that much interface circuitry is necessary for use with a print head. However, due to the delicacy of the assembly, it has been previously necessary in the art to locate this interface circuitry at some place relatively remote from the print head. This has associated problems, including the possibility of extra noise entering the system in the relatively long interface length between the two structures.
The present invention obviates these problems in a new way, not in any way taught or suggested by the prior art.
One attempt to obviate the above problems is exemplified by the Japanese Patent document No. 61-57357. FIG. 2 of this patent shows a three-piece thermal printing device which includes a circuit board as one element thereof. A top piece 16 and a bottom piece 9 have a circuit board 11 therebetween. A thermal head 8 is adapted to be located in contact with the circuit board. However, alignment of the structures in this patent would be a tedious manual process, which would be expected to be frequently necessary. The alignment would presumably be made by positioning the circuit board in the exact proper location and by tightening screws. However, the process of tightening the screws may itself misalign the circuit board. Moreover, it appears that this reference is restricted to the use of a circuit board/print head assembly which is one, pre-made integral piece. This would require the use of a specially-made part for this print head assembly. This would also require the entire print head assembly to be replaced if any component on the circuit board were to fail.
Finally, the shielding aspects of this reference are deficient. The teaching of this reference is that the circuit board should be totally located under the mounting means 16. This would require either a very large top plate or very little circuitry. Moreover, this would require low-height circuitry, and probably could not accommodate normal circuitry. In contrast, the design of the present invention enables more circuitry to be provided on the circuit board, because the circuit board can extend above the bottom plane of the top plate.
U.S. Pat. No. 4,571,598 also teaches a system with a circuit board that uses a thermal head. However, this patent adheres the circuit board to the substrate using glue or the like. This certainly does not obviate the problems associated with such assembly, however, as a tedious step in the process would include gluing the circuit board. Moreover, thermal expansion and contraction would be expected to eventually move the glued structures out of alignment, at which time an entire new circuit would
FIG. 8 of U.S. Pat. No. 4,684,960 shows a thermal print head assembly including a circuit board. Once again this circuit board/print head is a single piece assembly. Similarly, none of the other prior art in any way teaches or suggests the use of the structure according to the present invention, and therefore is relatively disadvantageous as compared therewith.
The print head assembly of the present invention obviates these problems by forming a base with a registry means which aligns the structures including the circuit board and a connector thereto in self-alignment. This base also forms a ground plane used for shielding the assembly. The base includes a top plane and a front edge extending above the top plane as well as including a pair of side surfaces. The base also includes the registry means which have been previously discussed, and which are preferably formed by at least two pins, extending upwardly. A thermal print head is located on the base with its connector at one end and with its printing elements at another end. The printing elements are aligned against the front edge surface of the base which includes a lip for that purpose. This aligns the thermal print head on the base. A controlling circuit board is also aligned on the base, by forming the controlling circuit board with holes that align with the registry means. These holes are preferably plated-through holes, so that the ground plane of the circuit board becomes electrically connected to the base through these registry means passing through the plated-through holes. The circuit board also has a connector disposed adjacent the connector of the thermal print head. A connecting structure is then aligned on the registry means and connects the circuit board connector with the connector of the thermal print head. Finally, a cover is provided for holding all of the structures together, and for compressing the structure in the area of the connection between the print head and the circuit board. The cover is also connected to the ground plane to provide additional shielding.